62 M. Prevost oji a New Gyrogomte. 



the midst of clay, marbled with red and bluish colours. These 

 siliceous blocks, which are often hollow in their interior, are 

 filled with the same clay, and with an immense quantity of 

 G. medicag^inula, and of the new species. In that case, the 

 fossils are free, and it is easy to separate them from the clay by 

 washing. The residuum obtained by this operation, appears to 

 the naked eye to be nothing but a very fine sand ; but, with the 

 assistance of a lens, it is distinctly seen that each grain is a part or 

 a complete mould of one of the two gyrogonites, or a broken 

 fragment of stems, the structure of which differs in no way 

 from that of the stems of the genus Chara. Nitric acid pro- 

 duces no effect upon these parts, which renders it probable 

 that they have been transformed into silex. When nitric 

 acid is poured upon the dried, and even the fresh, capsules of 

 recent chara, it causes a lively effervescence, produced by the 

 decomposition of a great quantity of carbonate of lime, which is 

 contained in the outer envelope of the capsule, as well as in 

 the stems. This effervescence destroys the opaque part of the 

 envelope ; and the nucleus, which is almost, in every respect, si- 

 milar to those that have become fossil, remains untouched in the 

 liquid. It ought to be remarked, on this occasion, that, in the 

 calcareous rocks which contain fossil gyrogonites, it is the outer 

 envelope that has been preserved, while the nucleus has disap- 

 peared, — a result the reverse of what the siliceous rocks dis- 

 close. 



M . C. Prevost found the same gyrogonite in the fresh-water 

 flints of Nogent le Rotrou, which were given him by M. J. 

 Desnoyers, who has also some specimens of a greenish compact 

 limestone, resembling, in its mineralogical appearance, some 

 beds of Jura limestone, of the Department de la Manche ; in 

 which country the specimens were found in digging wells. They 

 contain a globular gyrogonite, which differs, in some points, 

 from the G. medicaginula. Some fresh- water marls of the vi- 

 cinity of Epernay, collected by M. Deshayes, are filled with 

 gyrogonites which are also globular, but less perfectly so, and 

 larger than the G. medicaginula, and, perhaps, similar to those 

 already pointed out by M. Bigot de Morogues. There has also 

 been observed in the deposits, superior to the Fountainbleau 

 sandstone, above Valvin, a constant variety of elongated gyro- 



